The full story of directives

[wonderfan]

• The ngInit directive allows you to evaluate an expression in the current scope.

[wonderfan]

var ngInitDirective = ngDirective({
  priority: 450,
  compile: function() {
    return {
      pre: function(scope, element, attrs) {
        scope.$eval(attrs.ngInit);
      }
    };
  }
});

create one simple object with some attributes and pass it to ngDirective function

[wonderfan]

function publishExternalAPI(angular){
  extend(angular, {
    'bootstrap': bootstrap,
    'copy': copy,
    'extend': extend,
    'equals': equals,
    'element': jqLite,
    'forEach': forEach,
    'injector': createInjector,
    'noop':noop,
    'bind':bind,
    'toJson': toJson,
    'fromJson': fromJson,
    'identity':identity,
    'isUndefined': isUndefined,
    'isDefined': isDefined,
    'isString': isString,
    'isFunction': isFunction,
    'isObject': isObject,
    'isNumber': isNumber,
    'isElement': isElement,
    'isArray': isArray,
    'version': version,
    'isDate': isDate,
    'lowercase': lowercase,
    'uppercase': uppercase,
    'callbacks': {counter: 0},
    '$$minErr': minErr,
    '$$csp': csp
  });

[wonderfan]

      $provide.provider({
        $anchorScroll: $AnchorScrollProvider,
        $animate: $AnimateProvider,
        $browser: $BrowserProvider,
        $cacheFactory: $CacheFactoryProvider,
        $controller: $ControllerProvider,
        $document: $DocumentProvider,
        $exceptionHandler: $ExceptionHandlerProvider,
        $filter: $FilterProvider,
        $interpolate: $InterpolateProvider,
        $interval: $IntervalProvider,
        $http: $HttpProvider,
        $httpBackend: $HttpBackendProvider,
        $location: $LocationProvider,
        $log: $LogProvider,
        $parse: $ParseProvider,
        $rootScope: $RootScopeProvider,
        $q: $QProvider,
        $sce: $SceProvider,
        $sceDelegate: $SceDelegateProvider,
        $sniffer: $SnifferProvider,
        $templateCache: $TemplateCacheProvider,
        $timeout: $TimeoutProvider,
        $window: $WindowProvider,
        $$rAF: $$RAFProvider,
        $$asyncCallback : $$AsyncCallbackProvider
      });

[wonderfan]

      $provide.provider('$compile', $CompileProvider).
        directive({
            a: htmlAnchorDirective,
            input: inputDirective,
            textarea: inputDirective,
            form: formDirective,
            script: scriptDirective,
            select: selectDirective,
            style: styleDirective,
            option: optionDirective,
            ngBind: ngBindDirective,
            ngBindHtml: ngBindHtmlDirective,
            ngBindTemplate: ngBindTemplateDirective,
            ngClass: ngClassDirective,
            ngClassEven: ngClassEvenDirective,
            ngClassOdd: ngClassOddDirective,
            ngCloak: ngCloakDirective,
            ngController: ngControllerDirective,
            ngForm: ngFormDirective,
            ngHide: ngHideDirective,
            ngIf: ngIfDirective,
            ngInclude: ngIncludeDirective,
            ngInit: ngInitDirective,
            ngNonBindable: ngNonBindableDirective,
            ngPluralize: ngPluralizeDirective,
            ngRepeat: ngRepeatDirective,
            ngShow: ngShowDirective,
            ngStyle: ngStyleDirective,
            ngSwitch: ngSwitchDirective,
            ngSwitchWhen: ngSwitchWhenDirective,
            ngSwitchDefault: ngSwitchDefaultDirective,
            ngOptions: ngOptionsDirective,
            ngTransclude: ngTranscludeDirective,
            ngModel: ngModelDirective,
            ngList: ngListDirective,
            ngChange: ngChangeDirective,
            required: requiredDirective,
            ngRequired: requiredDirective,
            ngValue: ngValueDirective
        }).
        directive({
          ngInclude: ngIncludeFillContentDirective
        }).
        directive(ngAttributeAliasDirectives).
        directive(ngEventDirectives);

[wonderfan]

function ngDirective(directive) {
  if (isFunction(directive)) {
    directive = {
      link: directive
    };
  }
  directive.restrict = directive.restrict || 'AC';
  return valueFn(directive);
}

if the argument is function type,wrap it as link object, then return it as function

[wonderfan]

          controller: invokeLater('$controllerProvider', 'register'),

          /**
           * @ngdoc method
           * @name angular.Module#directive
           * @module ng
           * @param {string|Object} name Directive name, or an object map of directives where the
           *    keys are the names and the values are the factories.
           * @param {Function} directiveFactory Factory function for creating new instance of
           * directives.
           * @description
           * See {@link ng.$compileProvider#directive $compileProvider.directive()}.
           */
          directive: invokeLater('$compileProvider', 'directive'),

          /**
           * @ngdoc method
           * @name angular.Module#config
           * @module ng
           * @param {Function} configFn Execute this function on module load. Useful for service
           *    configuration.
           * @description
           * Use this method to register work which needs to be performed on module loading.
           */
          config: config,

The code snippet explains why you can register controller and directive in your module definition